A turbine machine can be taken as already known from the prior art and includes at least a rotor provided with rotor blades and a rotor housing surrounding at least in sections the rotor or the rotor blades, forming a rotor gap. The efficiency and the pump behavior of the turbine machine are determined by the rotor gap between the tips of the rotor blades and the wall of the rotor housing surrounding the rotor or the rotor blades. This causes pressure drops, which reduce the efficiency and pump clearance distance. The rotor gap is also affected by centrifugal force and thermal effects, dependent upon rpm. While the diameter of the rotor or the rotor blades varies directly with centrifugal force, the rotor and the rotor housing react with different time constants to temperature changes in the medium flowing through the turbine machine. This leads to complex changes in the rotor gap as a function of the rotor rpm, temperature, and time. With turbine machines constructed as aircraft turbines, the smallest rotor gaps, for example, exist with the so-called “hot re-slams”, in which a previously decelerated rotor is brought rapidly once more to maximum rpm after a short time. The rotor, still hot, thus has a thermally and centrifugal-force-conditioned maximum diameter, whereas the rotor housing, due to its lower thermal capacity, has already cooled off and is thus in a contracted state. Further factors influencing the rotor gap are, for instance, deformations of the rotor housing and radial motions of the rotor due to acceleration forces affecting the turbine machine. Altogether, therefore, in the construction of such turbine machines, a sufficiently large rotor gap must be maintained in the area of the rotor blade tips, for which reason the turbine machine mainly cannot operate at optimal efficiency and pump limiting distance. When testing turbine machines, the rotor gap therefore represents a central measurement value, so that turbine machines additionally include a gap-measuring system during testing in order to determine this rotor gap. What is more, the gap-measuring system is coupled to a capacitive sensor device, which has an electrode and a counterelectrode as capacitor for determining the measured capacitance values characterizing the rotor gap. In this, the rotor and its rotor blades are connected as an electrode of the sensor device. The counterelectrode of the capacitor is usually constructed as a sensor and is disposed inside the rotor housing. The measured capacitance values of the sensor device consequently change as the rotor blades run past it during operation, whereby the change in the measured capacitance values corresponds to the rotor gap. To measure the capacitance values, different electronic structural elements and methods are available to the expert, which operate using constant or alternating voltage and form the gap-measuring system. The gap-measuring system consequently receives a signal which changes frequency with the changing frequency of the blade, in which the amplitude stroke of each period corresponds to the minimum rotor gap of the blade passing under the sensor at that instant. The correlation between the amplitude stroke and the rotor gap is specified in advance by means of a calibration measurement and can be designed, for example, as a family of curves.
An alternative gap-measuring system or method known from the prior art operates with sensor devices which produce eddy currents in the rotor blades running past, which currents in turn react upon the sensor devices. The size of the effect is again dependent on the size of the rotor gap.
A further gap-measuring system or method known from the prior art operates with microwaves. Here the sensor device forms a resonator in the rotor housing, whose properties likewise change with the rotor blades running past and produce a measurement effect.
For all gap-measuring systems or methods, their use in series-produced turbine machines represents a far greater challenge than their use during testing of the turbine machines. Service lives of 10,000 hours or more are required, in which the sensor device must deliver reliable rotor-gap measurements. At the least, the sensor device is thus exposed, depending on the actual embodiment of the turbine machine, to varying temperatures up to 700° C., high pressures, oscillations, and further stresses due to water, salt, oil, dirt, metal abrasion, and the like.
A drawback of the known turbine machines or gap-measuring systems is therefore seen to be the fact that the counterelectrode of the sensor device must be arranged set back somewhat from the wall of the rotor housing that faces the rotor blades, so that in the event of a rotor-blade run-in, it will not be damaged. These setbacks are, however, undesirable because they lead to flow disruption. Alternatively, sensor devices based on eddy currents are known from the literature, which are designed so that they measure through the rotor housing against the rotor blades (so-called “through-the-case” sensors). For this, the rotor housing must be made thinner in the area of the sensor device. This method, though, leads to a considerable degradation of the measurement signal.